With the miniaturization and the high degree of integration of semiconductor integrated circuits, there is the demand to increase the resolution in the projection exposure device for the manufacture thereof. To meet this demand, a shortening of the wavelength of the light source for the exposure is promoted, and extreme ultraviolet light source devices (in the following also referred to as EUV light source devices') emitting extreme ultraviolet radiation (in the following also referred to as ‘EUV (extreme ultra violet’) with a wavelength of 13 to 14 nm and in particular 13.5 nm are developed as light sources for the exposure of next generation semiconductors.
Several methods for the generation of EUV radiation in EUV light source devices are known, and one method among these is to generate a high temperature plasma by heating and exciting an EUV radiation seed and to extract the EUV radiation which is emitted from this plasma.
One of the types of EUV light source devices employing such a method is the DPP (discharge produced plasma)-type EUV light source device. The DPP-type EUV light source device utilizes the EUV radiation from high temperature plasma generated by a current drive.
In EUV light source devices, Li (lithium) and Sn (tin) have drawn attention as the radiation seed, that is, the high temperature plasma raw material for the EUV generation, to emit EUV radiation with a wavelength of 13.5 nm at a high radiation intensity.
In the following, the mechanism of the EUV radiation on the basis of the DPP method will be explained briefly.
In the DPP method, for example a gaseous high temperature plasma raw material atmosphere is provided in the interior of a discharge vessel wherein electrodes are arranged, a discharge is generated between the electrodes in said atmosphere and an initial plasma is generated. Said initial plasma is constricted by means of the self magnetic field of the direct current flowing between the electrodes because of the discharge. Thus the density of the initial plasma becomes high and the plasma temperature increases abruptly. This effect is referred to in the following as pinch effect. By means of the heating because of the pinch effect the ion density of the plasma having reached a high temperature reaches 1017 to 1020 cm−3, and the electron temperature reaches about 20 to 30 eV, and EUV radiation is emitted from this high temperature plasma.
Recently, a method wherein solid or liquid tin or lithium having been provided to the surfaces of the discharge generating electrodes is gasified by an irradiation with an energy beam such as a laser and afterwards a high temperature plasma is generated by a discharge has been suggested for the DPP method in patent literature example 1. In the following, a case in which the energy beam is a laser will be explained. This method described below will be referred to as LAGDPP (laser assisted gas discharge produced plasma) method.
In the following, an LAGDPP-type EUV light source device will be explained using FIG. 11.
The EUV light source device shown in FIG. 11 has a chamber 1 being the discharge vessel. This chamber 1 is divided largely into two spaces by a partition wall 1c having an opening. In one of the spaces a discharge part is arranged. The discharge part is a heating and exciting means to heat and excite a high temperature plasma raw material containing an EUV radiation seed. The discharge part comprises a pair of electrodes 11, 12.
In the other space an EUV collecting mirror 2 to collect EUV radiation emitted from the high temperature plasma having been generated by heating and exciting the high temperature plasma raw material and to direct it from an EUV radiation extraction part 7 provided at the chamber 1 to an irradiation optical system of the exposure device not illustrated in the drawing, and a debris trap 3 to suppress a transfer of debris occurring as a result of the plasma generation by means of a discharge to the EUV radiation collection part are arranged.
The reference numerals 11 and 12 refer to disc-shaped electrodes. These electrodes 11, 12 are separated by a specified distance and rotate around a rotating shaft 16c, 16c by rotating a rotation motor 16a, 16b respectively.
The reference numeral 14 refers to a high temperature plasma raw material emitting EUV radiation with a wavelength of 13.5 nm. This high temperature plasma raw material 14 is a heated melted metal such as, for example, liquid tin, which is accommodated in a container 15.
The electrodes 11, 12 are arranged such that a part thereof is immersed in the container 15 accommodating the high temperature plasma raw material 14. The liquid high temperature plasma raw material 14 having got onto the surface of the electrodes 11, 12 is transported into the discharge space by means of rotating the electrodes 11, 12. Laser light 17 from a laser source 17a is radiated to the high temperature plasma raw material 14 having been transported into the discharge space. This laser light 17 gasifies the irradiated high temperature plasma raw material 17. By applying a pulsed current from a current supply means 8 to the electrodes 11, 12 in this state, in which the high temperature plasma raw material 14 has been gasified by the irradiation with the laser light 17, a pulsed discharge is started between the electrodes 11, 12 and a plasma P is formed from the high temperature plasma raw material 14. When this plasma P is heated and excited by the large current flowing at the time of the discharge and reaches a high temperature, an EUV emission from this high temperature plasma is generated.
The EUV radiation emitted from the high temperature plasma P is collected by the EUV collector mirror 2 and is extracted from the EUV extraction part 7 to an exposure device not illustrated in the drawing.
Thus, the extreme ultraviolet radiation emitted from the high temperature plasma being the point of the light emission is collected by the collector mirror 2 and the extreme ultraviolet radiation is outputted from the opening (EUV extraction part) 7. Now, the collector mirror 2 comprises reflective shells reflecting the extreme ultraviolet radiation emitted from the high temperature plasma and a reflective shell holding structure holding these reflective shells in the light source device. In the following, this configuration will be referred to as collector mirror assembly 20.
As to the collector mirror assembly, a grazing incidence collector mirror assembly is known wherein a plurality of reflective shells with different diameters, which are shaped as ellipsoids of revolution or hyperboloids of revolution and which are overlying a rotational central axis such that the positions of the collecting point approximately coincide, are arranged in a nested fashion and are held by a holding structure. The reflective shells are elements for which a reflective layer reflecting extreme ultraviolet radiation is provided at the surface of one side of a base material. In consideration of the mechanical strength and the thermal conductivity a metal material is selected as the material for the base material of the reflective shells.
In patent literature example 1, nickel, aluminium, copper, and nickel-cobalt alloys are cited as preferred materials for the material of the collector mirror. Then, a material by means of which a high reflectivity with regard to extreme ultraviolet radiation, typically extreme ultraviolet radiation with a wavelength of 13.5 nm, is obtained, is selected for the reflective layer. In patent literature example 1, palladium, indium, platinum, molybdenum, rhodium and ruthenium are cited as preferred materials for the reflective layer.
The light generated from the high temperature plasma and high velocity ions etc. enter the collector mirror assembly, and a portion thereof is absorbed and becomes a thermal load. Therefore, the temperature of each portion of the collector mirror assembly rises during the lighting operation.
With this temperature rise the reflective shells making up the collector mirror assembly are deformed by heat and deviate from the ideal shape of the reflective surface. And with the temperature rise there is also a heat deformation of the reflective shell holding structure and the positional relation between each reflective shell and the point of the light emission deviates from the ideal condition. As a result the collector performance deteriorates, and in case of a significant temperature rise the deterioration of the reflective film by means of, e.g., an oxidation is promoted, the reflectivity of the extreme ultraviolet radiation decreases and the collector performance worsens.
In an exposure device using extreme ultraviolet radiation, the light source device and the exposure device main body are divided by a small opening (aperture, the EUV extraction part 7 in FIG. 11). The reasons for the provision of the aperture are the provision of a spatial separation to avoid a mutual interference of the vacuum states of the light source side chamber and the exposure device main body side chamber and the cut-off of unnecessary light which actually does not contribute to the exposure. Generally, the aperture is provided at a position at which the spatial distribution of the emission light from the collector mirror assembly is narrowest (the collecting point).
When the collector performance of the collector mirror assembly deteriorates, the spatial intensity distribution of the light at the position of the collecting point becomes broad, because of which even light which should actually pass is blocked by the aperture and, potentially, the power necessary for the exposure may not be obtained any longer. Then, the deterioration of the collector performance impairs the angular distribution of the light after the passage through the aperture which becomes a factor for the decrease of the exposure quality. Thus, the deterioration of the collector performance brings about a decrease of the performance as a light source device such as a reduction of the light source output or a reduction of the homogeneity of the distribution of the light intensity.
To suppress the temperature increase of the collector mirror assembly becoming the reason for the deterioration of the collector performance, a cooling channel for cooling the reflective shell is provided. In patent literature 2, for example, a collector mirror assembly is shown wherein a flow channel through which a fluid passes is provided at the non-reflective surface of the reflective shell and heat is removed by a flow of a cooling fluid such as water.